A laser beam welding method is used for a bonding between resin members. The method is, for example, disclosed in Japanese Patent Application Publications No. 2001-105500, No. 2001-71384, and No. S60-214931 (which corresponds to U.S. Pat. No. 4,636,609).
FIGS. 8A and 8B are a schematic view explaining a method for manufacturing a resin mold by using a conventional laser beam welding method. FIG. 8B is a cross sectional view along with a scanning direction. In the laser beam welding method, firstly, the first resin member 101 and the second resin member 102 are laminated. The first resin member 101 is capable of transmitting a laser beam 100. The second resin member is capable of absorbing the laser beam 100. Next, the laser beam 100 is irradiated from a first resin member side. The irradiated laser beam 100 transmits through the first resin member 100, and reaches an attached surface (i.e., an interface) between the first resin member 101 and the second resin member 102. The second resin member 102 absorbs energy of the laser beam 100. The second resin member 102 is heated and melted by the absorbed energy. The heat generated in the second resin member 102 conducts to the first resin member 101. The first resin member 101 is also melted by the conducted heat. Thus, a welding spot 104 is formed at the attached surface between the first and second resin members 101, 102. The laser beam 100 is scanned in a direction shown by an outline arrow in FIG. 8A. Therefore, the welding spot 104 is linearly linked. Accordingly, after welding, a welded linear trace is formed along with a scanning track. By using the laser beam welding, since the first and second resin members 101, 102 are diffused mutually at the welding spot 104, a strong bonding strength is obtained. Therefore, the laser beam welding method is used widely, for example, for manufacturing a resin mold such as a part of an automotive vehicle.
However, energy distribution of the laser beam 100 at the welding spot 104 is not homogeneous. Energy density of the laser beam 100 has a Gaussian distribution so that the energy density at the center portion becomes maximum, and the energy density becomes lower as it goes to the outside. Therefore, when the outer portion of the welding spot 104 is heated up to a predetermined temperature, the center portion of the welding spot 104 becomes over heat. Therefore, resin may be pyrolytically decomposed so that the center portion of the welding spot 104 is vaporized. Further, since melting amount of resin at the center portion is larger than that at the outer portion, shrinkage may be generated after cooling. The vaporization of resin and the generation of shrinkage cause a reduction of bonding strength.
In view of the above problem, laser equipment for improving an inhomogeneous energy distribution of the laser beam by using a kaleidoscope is disclosed in Japanese Patent Application Publication No. H2-266918. The kaleidoscope is made of metal, and has a cylindrical shape. The inner surface of the kaleidoscope is formed of a mirror surface. When the laser beam enters into the kaleidoscope, the laser beam is multiply reflected by the mirror surface. Because of the multiple reflections, the inhomogeneous energy distribution of the laser beam is improved. However, when the kaleidoscope is mounted on the laser equipment, cost of equipment becomes much larger. Thus, a manufacturing cost of the resin mold also becomes higher.